1. Field of the Invention
This invention relates generally to a system and method for discharging a high voltage battery on a vehicle and, more particularly, to a system and method for discharging a high voltage battery following a vehicle damaging event by automatically discharging each battery cell using a cell balancing circuit and the stored energy in the battery.
2. Discussion of the Related Art
Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as the extended range electric vehicles (EREV), that combine a battery and a main power source, such as an internal combustion engine, fuel cell systems, etc., and electric only vehicles, such as the battery electric vehicles (BEV). These batteries can be different battery types, such as lithium-ion, nickel metal hydride, lead-acid, etc. A typical high voltage battery system for an electric vehicle may include several battery cells or modules to provide the vehicle power and energy requirements, where each battery module may include a certain number of battery cells, such as twelve cells. Different vehicle designs include different battery designs that employ various trade-offs and advantages for a particular application.
During a vehicle crash or other vehicle damaging event, the rearrangement of and/or damage to various vehicle parts and the like may cause unwanted electrical connections and the discharge of various fluids from the vehicle to occur. Because of the significant electrical power that may be stored in the vehicle battery, these things could cause potentially hazardous situations, such as electrifying of vehicle parts, chemical hazards, fire hazards, etc. The design of a vehicle battery system needs to consider these possibly harmful events.
Fault isolation detection systems are sometimes employed in electrical circuits to provide electrical fault detection. Electric vehicles are one electrical system that typically employ fault isolation detection systems to prevent a person from being harmed by high voltage on the vehicle.
In order to provide electrical fault isolation, some vehicles are equipped with a battery disconnect unit (BDU) that automatically disconnects or removes battery power from the vehicle after a crash or other event by opening battery contactors. Also, it is known in the art to employ a manual service disconnect (MSD), which is a device that separates a battery into two parts, where a trained responder responding to a vehicle crash could remove the MSD to isolate the battery. However, these known isolation techniques do not remove the charge from the battery, only contain it.
As mentioned, when a hybrid electric vehicle experiences a crash or other similar damaging event, the integrity of the vehicle battery or other rechargeable energy storage system (RESS) becomes a concern regardless of the degree of vehicle damage. It may be desirable to discharge the battery during such an event. Known battery discharge systems can be expensive, for example, thousands of dollars, and are typically heavy and require significant space. One known battery discharge system is a stand alone, embedded advanced monitoring system with DC/DC buck/boost converters including liquid cooled load banks and semi-advanced control systems. Also, compatibility between the various forms of energy storage devices in the market makes battery discharge a complicated task because of the various voltages, chemistries and current levels.
In one particular electric vehicle crash event scenario, the vehicle maybe severely damaged or “totaled” where it cannot be driven and may not be worth fixing, where the vehicle is taken to a scrap yard or other storage facility. As discussed above, if the vehicle battery still maintains significant charge, that charge may provide a potential risk for fire or other hazards. Particularly, it has been shown that during such a catastrophic vehicle crash event, short circuits in the battery circuit may cause a potential fire risk when the scraped vehicle is being stored several days and even weeks after the actual event.
When a vehicle battery is relatively new, each cell in the battery typically operates at about the same level of performance, i.e., maximum charge or capacity. However, as the battery ages over time, each cell typically degrades in performance differently than the other cells, where the performance of the battery is limited by the performance of the lowest performing cell. Further, a battery cell or battery module in the battery may fail or may otherwise be limited in performance for other reasons, such as an internal short, loss of capacity, high resistance, high temperature, etc. Therefore, electric vehicles typically include cell balancing circuits and control algorithms that control the charging of the battery so that the state-of-charge (SOC) of the battery cells is maintained about the same.